1. Field of the Invention
The present invention relates to a method and a device for simultaneously measuring the flow rate and the depth of a river utilizing ultrasonic-measuring technology.
2. Description of the Prior Art
In the standard method of measuring the flow rate of a river, the rate is calculated by the equation: flow speed.times.area (V.times.S). Propeller or cup type flowmeters (called mechanical flowmeters), or other local flowmeters using other principles, are used in order to measure the flow speed perpendicular to the flow cross-sectional area of the river. Although, the total average flow speed of the water passing through the flow cross-section should be measured in order to determine the flow rate of a river, tin fact, the general procedure used is: the total cross-section (S) is divided into a plurality of partial cross-sections (S.sub.i); local average perpendicular flow speeds (V.sub.xi) are calculated by measuring flow speeds (V.sub.i) with a local flowmeter at a plurality of points in a vertical line in each partial cross-section (S.sub.i) to measure the average flow speed of each partial cross-section (S.sub.i); and .SIGMA.q.sub.i, the sum of partial flow rate q.sub.i =S.sub.i .times.V.sub.xi, is calculated to determine the flow rate of the river.
The number of local flow speed measuring points depends on the allowable error of a river flow rate measurement. FIG. 1 shows an example using a 9-point measuring method.
When the river is not deep, say .ltoreq.3 m, a flow speed measuring rod (1) with a flowmeter attached thereon is immersed into the river as shown in FIG. 2. The depth of the river is measured by reading a scale notched on the immersed flow speed measuring rod. That is, the rod also serves as a measuring rule. The measuring rod (1) can have various structures and there is a type in which the rod is stuck into the bottom of the river and the flowmeter is translated along the rod.
The disadvantages of the known methods of measuring the flowrate of a river with local flowmeters are as follows:
(1) It takes a lot of time to measure the local flow speeds at each point. Accordingly, it is time-consuming to measure the flow speeds at a plurality of points which is needed for improving the measuring accuracy of the average perpendicular flow speed (V.sub.x). Since the fluctuation of local speeds (V.sub.i) is high, an average value of the local flow speed (V.sub.i) should be taken, which can require 5-minutes of continuous measurement. Local flow speed measuring points are selected for relatively precise evaluation of the vertical flow speed distribution. For example, the positions of the measuring points of Vi for 5-point measurements are Ho, 0.2 H, 0.6 H, 0.8 H, and H. Ho and H are points as near as possible to the surface and the bottom of a river, respectively. As it takes time to move the flowmeter to individual points, the flow speed measuring productivity is very low. PA1 (2) In particular, accurate flow speed measurement is necessary at the bottom of a river because the flow speed distribution is complex. However, it is impossible for mechanical flowmeters to measure the flow speeds near the surface and bottom of a river because the diameter of the propeller or the cup-rotor of the mechanical flowmeters are relatively large. PA1 (1) It is almost impossible for the ultrasonic wave to be reflected without dispersion at a predetermined point (O) of the bottom of a river and reach the vibrator receiving the ultrasonic wave because the bottom is quite irregular. Thus, a reflecting plate should be placed on the bottom. PA1 (2) When measuring the flow speed using the difference method, there is an advantage in that by knowing only the distance L=AB, the average perpendicular speed (V.sub.x) can be calculated using the general expression .DELTA.t=2LV/C.sup.2, without the need for knowing the incident or reflecting angle (.THETA.) and the propagation distance (l.sub.1, l.sub.2) of the ultrasonic pulse with paths A.fwdarw.O.fwdarw.B or B.fwdarw.O.fwdarw.A. However, small errors in measuring the ultrasonic wave speed (C) results in large errors in calculating the flow speed. PA1 (3) When measuring the flow speed using the frequency difference method, i.e., using the general expression .DELTA.f=2V/L, the condition l.sub.1 =l.sub.2 =l should be obeyed and the reflection angle (.THETA.) should be known. However, accurate measurement of such data is quite difficult. PA1 (4) The distance L=AB should always be varied to ensure the optimum reflecting angle according to the change of the water depth (H).
FIG. 3 illustrates one method of perpendicular flow speed using ultrasonic waves. The flow speed measuring method using a reflected ultrasonic beam is performed using a pipe-type ultrasonic flowmeter, for example 990 DB developed by Controlotron, U.S.A. FIG. 3 shows vibrators (4, 4') transmitting and receiving ultrasonic waves (e.g., piezoelectric ceramic), and a support (5) locating and supporting the vibrators at respective points (A, B) on the surface of the water. Although the method shown in FIG. 3 can measure the perpendicular average flow speed (V.sub.x) perpendicular to the depth of water (H), the method has the following defects.
Besides the above-described defects, there are other defects which are common to any ultrasonic flow speed measuring method and are explained in the description of other methods.
The above-described method is beneficial when the water is deep, but because of its defects, has a disadvantage in that it is difficult to apply when the depth is relatively shallow (H.ltoreq.3 m).